Magnetic recording media generally comprise a magnetic layer coated onto at least one side of a nonmagnetizable substrate. For particulate magnetic recording media, the magnetic layer comprises a magnetic pigment dispersed in a polymeric binder. The magnetic layer may also include other components such as lubricants, abrasives, thermal stabilizers, catalysts, crosslinkers, antioxidants, dispersants, wetting agents, fungicides, bactericides, surfactants, antistatic agents, nonmagnetic pigments, coating aids, and the like.
Some forms of magnetic recording media, such as magnetic recording tape, may also have a backside coating applied to the other side of the nonmagnetizable substrate in order to improve the durability, conductivity, and tracking characteristics of the media. The backside coating also includes a polymeric binder and other components such as lubricants, abrasives, thermal stabilizers, catalysts, crosslinkers, antioxidants, dispersants, wetting agents, fungicides, bactericides, surfactants, antistatic agents, nonmagnetic pigments, coating aids, and the like.
The polymeric binders of the magnetic layer and the backside coating are commonly derived from polymers which require curing in order to provide magnetic recording media with appropriate physical and electromagnetic properties. To prepare such media, the components of the magnetic layer or the backside coating, as appropriate, are combined with a suitable solvent and thoroughly mixed to form a homogeneous dispersion. The resulting dispersion is then coated onto the nonmagnetizable substrate, after which the coating is dried, calendered if desired, and then cured.
The polymeric binders of magnetic recording media are most commonly prepared from polymer blends comprising a hard component, i.e., a polymer with relatively high glass transition temperature and modulus, and a soft component, i.e. a polymer with relatively low glass transition temperature and modulus. In the past, polyurethane polymers have been widely used as the soft component.
Copolymers based on vinyl chloride or vinylidene chloride have been widely used as the hard component of choice for use with polyurethanes, due to their miscibility and compatibility with polyurethanes and their relatively high glass transition temperatures, modulii, and hardness. For example, Japanese Publication No. JP61-26132 describes the use of vinyl chloride/vinyl acetate/vinyl alcohol copolymers as a polymeric binder component in magnetic recording media.
Magnetic pigments tend to agglomerate and can be difficult to initially disperse in the polymeric binder or be difficult to keep dispersed in the polymeric binder over time. Low molecular weight wetting agents, or dispersants, are often employed to facilitate such dispersion. For higher pigment loadings, i.e., the use of greater amounts by weight of magnetic pigment, greater amounts of wetting agent or dispersant may be required. This is not always desirable. Dispersants tend to plasticize binder systems and decrease their modulus. Further, excess dispersant may exude from a cured binder system over time, leading to changes in the properties of the media as well as to contamination of a recording head or the like.
To help alleviate the problems associated with added low molecular weight dispersants or wetting agents, polymeric binders formed from "self-wetting" polymers have been developed. "Self-wetting" polymers have dispersing groups pendant from the polymer backbone that help disperse the magnetic pigment. Representative examples of dispersing groups include quaternary ammonium, amine, heterocyclic moieties, salts or acids based on sulfate, salts or acids based on sulfonate, salts or acids based on phosphate, salts or acids based on phosphonate, salts or acids based on carboxyl, mixtures thereof, and the like. As a result of using self-wetting polymers, less low molecular weight dispersant or wetting agent, or even no low molecular weight dispersant or wetting agent, may be needed to disperse the magnetic and nonmagnetic (if any) pigments in the polymeric binder. Self-wetting vinyl chloride copolymers have been described. See, e.g., U.S. Pat. Nos. 5,139,892; 5,126,202; 5,098,783; 5,064,730; 5,028,676; 5,008,357; 4,861,683; 4,784,913; 4,770,941; and 4,244,987.
Vinyl chloride or vinylidene chloride copolymers, however, tend to degrade over time, releasing gaseous HCl which can change the properties of the media as well as corrode the recording head or the like. Accordingly, some investigators have described vinyl copolymers used in magnetic recording media, wherein the use of vinyl chloride and vinylidene chloride has been avoided. See, e.g., U.S. Pat. Nos. 5,098,783; 4,876,149; and 4,837,082; and Japanese Publication Nos. SHO 62-0162; SHO 54-84708; SHO 54-46519; and SHO 54-46518.
What is still needed in the art, however, is a hard component for use in magnetic recording media which imparts high modulus and high resilience to magnetic recording media, but which does not contain any vinyl chloride or vinylidene chloride components.